Over torqueing is the most common cause of gaskets leaking. It distorts the metal which needs to be placing even pressure on the gasket into higher and lower pressure areas.. sometimes even to zero on the lower side.... and when dealing with aluminum blocks or heads it only takes stripping out one hole in a bad location to more than justify buying and using a torque wrench.
It is also important to tighten up the bolts in increments and a back and forth pattern like with wheel nuts (lugs). It is even more important with light metal objects such as oil pans....